1. Field of the Invention
The invention pertains to the field of variable cam timing systems. More particularly, the invention pertains to a method of reducing rotational oscillation of a vane style phaser with a center mounted spool valve.
2. Description of Related Art
Internal combustion engines have employed various mechanisms to vary the angle between the camshaft and the crankshaft for improved engine performance or reduced emissions. The majority of these variable camshaft timing (VCT) mechanisms use one or more xe2x80x9cvane phasersxe2x80x9d on the engine camshaft (or camshafts, in a multiple-camshaft engine). In most cases, the phasers have a rotor with one or more vanes, mounted to the end of the camshaft, surrounded by a housing with the vane chambers into which the vanes fit. It is possible to have the vanes mounted to the housing, and the chambers in the rotor, as well. The housing""s outer circumference forms the sprocket, pulley or gear accepting drive force through a chain, belt or gears, usually from the camshaft, or possibly from another camshaft in a multiple-cam engine.
Since the phasers cannot be perfectly sealed they are subject to oil loss through leakage, especially in vane phasers that use the conventional four-way valves mounted in the valve body to control the phaser. Four-way valves have a supply and a return port and at least two ports, which go to the load, or in the case of the phaser, to the advance and retard chambers. The number of lands on the spool may vary depending on need. Typically, the four-way valve is remotely located and has many leak paths that can cause the phaser to move back and forth in reaction to the torsionals of the camshaft. Because the phaser is not 100% sealed and prone to leakage the four-way valve must have an open null position to makeup for the oil that has leaked from either the advance or retard chamber. The open-null position allows oil to leak into the inlet lines leading to the advance and retard chambers. In turn, the open null position of the four-way valve also increases the positional oscillation of the camshaft due to the additional leak now present between the advance and retard chambers.
In response to the increased amount of leakage that occurs with the use of a conventional four-way valve remotely, the four-way valve has been moved to the center of the phaser, which is conventionally in the rotor, but may be elsewhere. By moving the four-way valve (spool valve) to the center of the rotor many of the leaks paths that were originally present such as the cam bearing, the nose oil feed bearing, and the spool housing to engine block interference are eliminated. The main sources of leakage that are still present are from chamber to chamber, chamber to atmosphere and across the spool in particular at the open null position. Modifying the spool valve to a closed null design could further reduce leakage. However, if the null oil flow is completely shut off, the oil that does leak out of the rotor increases the positional oscillations that take place. Therefore, there is a need in the art to reduce the leak paths, preferably by using a closed null spool design, and provide makeup oil to the chambers of the phaser to reduce positional oscillation via some other means.
A phaser for an internal combustion engine having at least one camshaft. The phaser has a housing and a rotor. The housing has an outer circumference for accepting a drive force and the rotor connects to a camshaft coaxially located within the housing. The housing and the rotor define at least one vane separating chambers, advance and retard. The vane shifts the relative angular position of the housing and the rotor. The phaser also includes a spool valve comprising a spool slidably mounted within in a bore in the rotor. The spool routes operating fluid from a supply of pressurized fluid to the chambers. At least one passage from the supply to the chambers provides makeup fluid. The passage includes a check valve and a restrictor.